The invention relates to a device and to a method for contacting work that is to be subjected to electrolytic treatment, more specifically work to be electroplated, in electrolytic processes. This method includes but is not limited to electrolytic plating and electrolytic etching of printed circuit boards.
The surface of the work to be electrolytically treated is electrically conductive. The surface of work which is made within of an electrically non-conductive material is usually only provided with a very thin metal layer that is highly sensitive to mechanical stresses and is electrolytically treated. During electroplating, said surface is cathodically polarized, i.e., the surface is connected to the negative pole of a direct current source. Accordingly, the counter electrode, an anode in this case, is connected to the positive pole so as to be electrically conductive. For electrolytic etching, the polarities are inverted. The work is then under anodic polarization.
Current is supplied to the work in varied manners. In dip tanks, so-called racks or rotating clamp-racks are being used. The work is usually retained by springable and electrically conductive contact bows, attachment screws or wires which also serve to supply the current. The racks are adapted to the work.
For fastening board-shaped work to a flight bar, clamps are used as well. Clamps made from metal are widely used in the printed circuit board technique in particular. They are arranged in one row, several centimeters apart, on the flight bar. To place the work on the rack, the clamps are opened. They grasp the boards by way of a spring and carry them during wet-chemical treatment. Concurrently, the clamps serve to supply the current.
In continuous processing plants, more specifically in the printed circuit board technique, the current is also, i.a., introduced into the printed circuit board by means of clamps. Concurrently, the clamps serve to convey the printed circuit boards through the continuous processing plant.
DE 36 12 220 C2 describes a clamp for clampingly holding boards or board-shaped parts such as printed circuit boards for electroplating them in dip tanks. In this document, the clamp is called holding tongs. Clamping projections provided on the holding tongs are placed against the surface of the work for electrical contact.
EP-A 0 254 962 shows, i.a., a clamp for holding printed circuit boards for treatment in a horizontal continuous processing plant. In this case as well, clamping jaws are placed against the printed circuit board. The contact areas of the clamping jaws in this case not acute but plane.
Furthermore, DE-OS 25 45 906 discloses a holding device for parts to be coated in electroplating baths. The device comprises a metal frame and an impervious outer sheath that covers the frame except for at least portion of the frame that remains bare so that electrical contact is possible between the part to be retained by the holding device and a contact member that encompasses the uncovered part. The contact member is made of a non-conductive, resiliently compressible supporting mass containing conductive particles. The mass is electrically non-conductive when it is not compressed. Electrical conductivity is only obtained when the material is compressed by attaching the parts to be treated.
DD-WP 71920 mentions a suspension device for work to be electrolytically treated that is provided with a coating made from silicone resin containing a certain metallic powder in high proportion. In said document, this embodiment is considered disadvantageous as the coating is complicated and expensive to manufacture and only lasts a short time in industrial utilization so that it often needs to be renewed.
The fine line printed circuit board technique requires increasingly thin layers of copper laminate. The purpose thereof is that, after electroplating the printed circuit boards, the fine circuit traces are prevented from being undercut in the subsequent etching procedure. Therefore, copper base layers of e.g., only 5 μm thick are used. In the SBU printed circuit board technique (SBU: Sequential Build Up), the layers to be electrolytically strengthened have a thickness of less than 1 μm. In the existing plants, the current intensity in the clamp ranges from 20 A to 50 A on each side of the printed circuit board. The use of the reverse pulse current technique renders the process more difficult. At a metal deposition speed which may be compared to that in use in the direct current technique, the effective output to be transmitted through each contact is still about 25% higher because of the metal dissolving reverse current. Under such extreme requirements, the known clamp contacts fail. This applies to the printed circuit board technique in both dip tanks and continuous processing plants. It also applies to the electroplating of other plastic parts by means of racks in dip tanks under comparable conditions.
The object of the present invention is therefore to overcome the disadvantages of the known methods and devices and more specifically to allow very thin, electrically conductive layers to be electrically contacted and electrolytically treated, particularly with high electrolytic current. This also includes the object to treat thin, conductive layers according to the so-called direct metallization method.
The solution to this object is achieved by the device and the method below. Advantageous developments of the invention are recited in the subordinate claims.
The device and the method in accordance with the invention serve to place electrolytically to be treated work in electric contact. For making electric contact, the device comprises contact carriers with contact elements for supplying the current to the work, more specifically to the work to be electroplated. According to the invention, at least the contact areas of the contact elements that can be placed in electric contact with the work are made from an elastic material that must moreover be electrically conductive. Clamps, tongs, clips, bows and springs with one or two mobile legs are suited as contact carriers in the first place. Mechanical tolerances in the contact elements which are lying plane on the work are compensated through electrically conductive surfaces made from the elastic material in such a manner that electrolytic treatment current, more specifically electroplating current, is transmitted over the entire area of the contact element.
With a device for clamping and electrically contacting the work with the elastic contact material which, at the same time, is provided with very good conductive properties, the mechanical tolerances always encountered in actually plane-parallel planar contact elements are completely compensated when the work is being grasped by means of the clamps, clips and so on mentioned. Deformations of the work such as outward deflections on printed circuit boards or curvatures on molded goods are compensated as well. The closing pressure, and accordingly the pressure necessary to deform the elastic contact material in such a manner that the tolerances are compensated, can be generally provided by a spring. In contrast to known contacting facilities, the device in accordance with the invention moreover permits to reliably prevent the surface of the work from being mechanically damaged thanks to the large engaging area or to the many engaging points. Surprisingly, current transfer is thus ensured in all cases without any problem.
With the device described in DE 36 12 220 C2, by contrast, sensitive layers on the surface of work to be electroplated may easily be damaged. As a result thereof, the conductive layer on the surface is interrupted which causes the current conducted from the clamp to the work to be hindered, or altogether interrupted. If, moreover, boards of varied thickness are treated with the known device and are retained by the holding tongs for this purpose, the projections provided on the holding tongs possibly do not all touch the surfaces of the board in the same way so that in this case, the electric contact in which the board is placed is insufficient. Only if the work has a thickness adapted to given holding tongs, the four clamping projections will all rest in the same proper way on the surface of the board. This is also the case when, with other thicknesses, two respective clamping projections are allowed to dig in the surface. In this case, the surface is damaged and in case of thin electrically conductive surface layers like foils e.g., they can no longer be reliably placed in electric contact as a result thereof. Under these conditions, high electroplating currents will burn the contact points.
EP-A 0 254 962 claims to obviate the problems mentioned by providing a parallel guidance consisting of a driver pin and grooves which is intended to allow placement of the contacts against the entire surface. In practical operation however, this proved impossible because of the technically necessary clearance in the driver pin and of the process tolerances always encountered. At best, the clamping jaws will touch the work on a line, in fact only on one corner of their contact areas. A spacing due to tolerance between the contact area and the surface of the printed circuit board sized in the micrometer range already suffices to contribute no longer to the transfer of the high current required. Accordingly, the current density and, as a result thereof, the voltage drop on the contact resistance and the power loss on the small contact-making corner are extremely high. The contact point is hence strongly heated. If the electrically conductive layer on the surface of the work is thick enough, this heat can be dissipated. With printed circuit boards having the usual copper layers of 17 μm thick provided on the insulating laminates, this is still possible. In exceptional cases however, a burn through at the clamp may already occur at this layer thickness.
The afore mentioned drawbacks of the known devices are not encountered when using the device in accordance with the invention. Even materials with very thin electrically conductive layers can be treated with high current density without damage being caused to the layers.
In case of very thin electrically conductive layers that are to be electrolytically strengthened, it is advantageous when the transition line of the current supplying contact element between the edge of the contact and the work is particularly large. Accordingly, if a round contact element has a large circumference or if an oblong contact element is particularly long, a large current can be transmitted. The same is achieved when a multitude of points of a contact area reliably touch and thus contact the surface of the work. Again, this is only possible using an inherently elastic contact material. The current density in the contact element and in the region of the surrounding surface of the work is then within the permissible range. As a result thereof, burns are avoided when electric contact is being made. In case of large contact elements, mechanical tolerances however have a particularly great influence because of the large area of contact. Without using the contact element of the invention, only fractions of the area of the metallic contact would rest on the surface of the work so that current would be introduced into the work, entailing all of the above described drawbacks. These tolerances in the micrometer range also include the irregularities of the contact surfaces. Even if rigid contacts of large dimensions are resting in a plane-parallel manner, these irregularities prevent the current from being distributed evenly over the entire metallic contact area when the contact element of the invention is not being used.